This invention relates to a novel electrodeionization module adapted to transfer ions in a liquid under the influence of a polar field. More specifically, this invention relates to an electrodeionization apparatus adapted to purify aqueous liquids to effect the production of high purity water.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture.
The most well-known processes include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration and ion exchange. A lesser known method is electrodeionization, occasionally mistermed filled cell electrodialysis.
The first apparatus and method for treating liquids by electrodeionization was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an apparatus and process for the removal of ions within a liquid mixture in a depleting chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentrating chamber under the influence of an electrical potential which causes the preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transfer ions and carries them in concentrated form. The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for the movement of ions.
The term "electrodeionization" refers to the process wherein an ion exchange material is positioned between anion and cationic diaphragms. The term "electrodialysis" refers to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Illustrative of other prior art attempts to use the combination of electrodialysis and ion exchange materials or resins to purify saline from brackish are described in U.S. Pat. Nos. 2,794,770; 2,796,395; 2,947,688; 3,384,568; 2,923,674; 3,014,855 and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517 and 4,284,492.
A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745. The apparatus utilizes ion depleting compartments containing an ion exchange solid composition and a concentrating compartment which is free of ion exchange solid material. The electrodeionization apparatus includes two terminal electrode chambers containing an anode and a cathode respectively which are utilized to pass direct current transversely through the body of the apparatus containing a plurality of ion depleting compartments and ion concentrating compartments. In operation, the dissolved ionized salts of the liquid are transferred through the appropriate membranes from the ion depleting compartments to the ion concentrating compartments. The ions collected in the ion concentrating compartments are removed through discharge outlets and are directed to waste. One difficulty in utilizing electrodeionization apparatus is the deposit of insoluble scale within the cathode compartment primarily due to the presence of calcium, magnesium and bicarbonate ions in the liquid which contact the basic environment of the cathode compartment. Scaling can also occur in concentrating compartments under conditions of high water recovery. The scale comprises primarily alkaline earth metal carbonates and hydroxides which increase the electrical and hydrolytic resistance of the cathode an concentrating compartment and, as a result, lowers the efficiency of the apparatus.
It has been proposed in U.S. Pat. No. 3,341,441, in an electrodialysis process, to reverse periodically the direction of current flow in which case, the electrode once serving as the cathode becomes the anode and anode electrode becomes the cathode. The solution flowing through the anode chamber becomes acidic due to anodic electrolytic action, and the acid thus formed tends to dissolve a small portion of scale formed therein during the time the electrode was cathodic. In the process, the acid generated within the anode chamber is allowed to attain a sufficiently high concentration in the chamber so as to dissolve precipitated solid formed therein during the electrodes' previous cathodic cycle and thereafter, reversing the polarity of the direct current is performed at periodic intervals. In a preferred form of the process a third step is also employed comprising continuously flushing the cathode compartment with a sufficiently large volume of electrolyte solution to quickly remove any base generated therein. When the direct current is reversed, the ion depleting compartments become the ion concentrating compartments and the ion concentrating compartments become the ion depleting compartments. This process is undesirable since a large volume of liquid being purified must be discharged to waste in a time interval immediately following voltage polarity reversal since the concentration of electrolyte in the newly formed ion depleting compartments is too high for a period of time to render the purity of the liquid product acceptable. An additional disadvantage of this process is that in order to avoid scaling in the concentrating and cathode compartments, that the time between polarity reversal must be short, since the generation of hydroxide ion that occurs at the cathode and anion membranes in normal operation is unbuffered due to the lack of ion exchange resins in the compartments of the device.
Accordingly, it would be desirable to provide a means for preventing scale buildup in the electrode and concentrating compartments in an electrodeionization process while at the same time avoiding the loss of any liquid product which is being purified, and extending the time between reversal cycles as long as possible.